Mike,

In my experience, getting the bulk of the old pad off is sufficient. The "cleanest" I've gotten the underside of the hood before applying the new adhesive has been to the point where if I ran my fingers over the metal, I'd feel rough old adhesive and the "fuzz" of the old pad. Really no need to get it cleaner than that as the 3M adhesive, if applied properly, will work just fine.

Good luck-

Terry